Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Composite having voids in a component
Patent
1995-03-15
1996-03-05
Lusignan, Michael
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Composite having voids in a component
427244, 427369, 4283088, 4283193, 4283197, B32B 326
Patent
active
054966299
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a method of treating a porous material to modify a property thereof.
BACKGROUND ART
For certain applications, porous materials (e.g. paper) must be treated so as to impart a desired property thereto. For example, if the porous material is required to resist the penetration of liquids then a suitable coating or dipping treatment of the material may be required. Such dipping or coating treatments may also be required if it is desired to increase fire resistance or enhance strength of the material. Generally however such treatments require a relatively large amount of the treatment agent which might result in undesirable modification of other properties of the material (e.g. its air permeance).
A treatment which does not involve dipping or coating, and which is effected to improve the barrier performance of a porous material is disclosed in EP-A-0,272,798. In this process, a porous material is treated by applying a small pressure differential across the material, and treating the higher pressure side with a suspension, dispersion or aerosol of the pore modifying agent. As a result, the particulate pore modifying agent passes into the larger pores and is retained there to provide zones of high surface area which restrict the passage of micro-organisms through the material, thereby improving performance.
DISCLOSURE OF INVENTION
According to the present invention there is provided a method of treating a porous material comprising establishing a pressure differential across the material and treating the higher pressure side with a suspension, dispersion or aerosol of droplets of a liquid so that said droplets enter at least some of the pores; and wherein the treatment conditions are such that the aerodynamic forces within the pores exceed the surface tension forces maintaining integrity of the droplets which therefore spread to provide a surface coating within the pores. Note: by referring to `droplets of liquid` it is merely implied that the drops are of flowable and/or spreadable consistency. They may comprise liquid and/or semi-solid components, and may include solids in solution or in suspension.
In operation of the method, the flow of air (or other gas) through the material which is generated by the pressure differential causes the liquid droplets to enter the pores of the material just as the pore modifying agent enters the pores in the process described in EP-A0,272,798). Within a pore, the liquid droplets are (it is believed) retained on the pore surface by Van der Waal's forces. However, the conditions under which the method is operated are such that the aerodynamic forces generated within the pores by the gas flow overcome the surface tension which would otherwise cause the liquid to remain as droplets and thereby cause a "spreading" of the droplets on the pore surfaces to form the coating.
The method of the invention therefore results in a coating of the liquid being provided on the surfaces of the pores within the material. This coating will be of a high surface area (since the internal porous surfaces of the material will provide a high surface area on which the coating may be formed). The coating may be of extremely low thickness, e.g. of the order of a few molecules thick. As such, there is no substantial effect on the air permeance of the material (typically a decrease of less than 50% and preferably less than 20l % as measured in Bendtsen units).
The method of EP-A-0,272,798 is dedicated to producing deposits of particles which retain their particulate nature, so as to provide greatly increased surface area. Therefore the pore modifying agents used consist largely of solids, and they are deposited in the pores under gentle conditions, e.g. relying on Brownian motion capture. Thus the process of EP-A-0,272,798 provides a significant enhancement in barrier performance of this material. In contrast, the method of the present invention results in a coating on the pore surfaces and there is generally no improvement in barrier performance. In fact
REFERENCES:
patent: 4551191 (1985-11-01), Kock et al.
patent: 5004645 (1991-04-01), Tallentire et al.
patent: 5281437 (1994-01-01), Singh
Sinclair Colin S.
Tallentire Alan
Lusignan Michael
The Victoria University of Manchester
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